Google offers a variety of services related to searching and advertising. For example, a user may enter a search term on a web page offered by the service, and the service returns a webpage containing matching results.
The service may also display advertisements based on the search term. For example, users of Google's AdWords service may indicate an interest in having their advertisement displayed each time a user enters a particular search. These same advertisers may agree to pay a fee each time a user clicks on the search. Advertisers may limit the advertisement to certain time periods.
More than one advertiser may be interested a particular search term. Accordingly, Google uses a variety of data and techniques to determine which advertisement should be displayed and its placement of the webpage. These techniques include ranking advertisements based on the popularity of the advertiser's website and the prominence of the keyword on the advertiser's site.